Merceders

Last night the A/C blew cool air but not icey cold as usual, and the Engine temperature was high (90-100, sometimes 110).

My auxiliary fans just stop working, (no matter idling till 100C, A/C on and off ¡V compressor clutch is working, stop & go for 30 minutes)

I checked and clean the fuse ( D, red-30 Amp).
But how to trouble shoot the Relay?

What are the possible causes for auxiliary fans not kicking in?
- CT sensor (the green one on top of the engine)?
- Relay (what position?)

Appreciated you help!!

TomBeverlyHills

My mechanic just repaired a similar problem on my '92 300CE.

On my car, the two-speed aux fans should come at low speed anytime the A/C is on. When they're running at low speed, you can't even hear them.
Then when the temp gets to about 120C (I think) the aux fans come on at high speed (which you can definitely hear). My mechanic replaced a temp sensor switch that resides in the thermostat housing. I think if you unplug that switch and short the wires together and the fans come on, then the switch is probably bad. If you short the wires and the fans don't come on, then you probably have a relay out. If only one fan comes on, then I'd bet you have a bad fan.
I hope that helps.

Kestas

What about the fans themselves? They can be checked by applying 12v directly to the motor.

TomBeverlyHills

Kestas,
If it was my car, that's how I'd check the motors (by hooking them directly to 12V), but I've never had to do it so I'm not 100% sure that it wouldn't do any damage.
But taking little risks is part of what makes life fun.
TomBeverlyHills

Merceders

I have tried unplug the CT sensor wire, and the fan starts working LOUD! so should be the CT sensor then.
Should the fan be on if the relay is faulty?

Anyway I have dropped off my car to my mechanic, so I will know whats wrong and will post the result.

Thanks so much!

bamberger_1

Hi guys,
had the same problem and had it fixed. Non of the previously mentioned by the posters was the problem. The problem is that the refrigerant level is low and the pressure to activate the fan(s) is never reached. They are pressure activated. You can test them by bridging the pressure switch (red) on top of the dryer bottle, when bridged the fan(s) shall run. I say fan(s) since depending on the year of the car there either 2 fans or one fan with 2-speeds.
When outside temperatures reach 85+ the required refrigerant pressure cant be reached and the A/C goes on and off, but never gets really cold. Have the system evacuated, the dryer bottle replaced and a fresh refill of R134a and all is back to normal.

1stmbenz

I have 1993 400E. How do I check if my cooling system is operating at maximum efficiency. Is there a special test. Also, mentioned earlier, I think 120 degrees is too high. In my car thats the red line.

Merceders

For my auxiliary fans non-functioning problem,
My mechinic changed the auxiliary fans resistor as well as the auxiliary fans relay ( total parts & labor about US$80) , now the problem is fixed.

However, the tempature is now above 80-90C most of the time, a bit higher than before the fix.
Anyway I know the car runs fine even the temp reach 100C, so shouldn't be any problem.

In where I live (Hong Kong), summer temp can reach 30-35C (95F), with the A/C on and traffic jam everywhere. It is a must to keep the car cool
I will flush the Coolant this weekend, probably change the host & the thermostat as well, just to be on the safe side.


Is there any else preventive measures I can do to make the car running cool and away from overheating?
Or should I believe " If it arn't broken, don't fix it" approach??

Any Advice??